As reforming methods adopted in reformers used in fuel cell systems and the like there are known steam reforming, partial oxidation reforming and autothermal reforming methods. Above all, the autothermal reforming method is said to be shorter in starting time and faster in load following performance than in the conventional steam reforming method because there occurs a heat transfer between oxidative exotherm and reforming endotherm in the interior of the reforming catalyst used.
However, also in the autothermal reforming method, it is necessary to raise the temperature of the reforming catalyst up to the temperature at which the catalyst exhibits its fuel oxidizing activity. As to a reforming catalyst filled into a catalyst bed, if the reforming catalyst is heated strongly using a burner for raising the catalyst temperature to shorten the starting time, the temperature of only an inlet portion of the catalyst bed rises rapidly due to high-temperature gas issuing from the burner. Therefore, for making the temperature of the entire catalyst bed reach a reformable temperature, it is necessary to repeat ON-OFF of the burner.
According to a method involving introducing gas heated separately with a heater into a reforming catalyst bed and preheating the reforming catalyst up to a reformable temperature, not only much electric power is needed, but also it is difficult to shorten the starting time.
That is, for starting the autothermal reformer stably and quickly, it is absolutely necessary that the temperature of the reforming catalyst be raised quickly with a desired temperature gradient from the inlet toward the outlet of the reforming catalyst bed.
That is, in a steady-state autothermal reforming reaction, the inlet temperature of the catalyst bed is higher than the outlet temperature of the same layer, so in starting the autothermal reformer it is necessary to preheat the catalyst bed inlet to a higher temperature. With the conventional methods, this point is not attained to a satisfactory extent.
In the following Patent Literature 1 there is disclosed a method in an autothermal reactor wherein, in order to shorten the time until restarting after stop, the supply of air is stopped after stop of supply of both reforming fuel and water, allowing the catalyst bed temperature to be retained using only a partial oxidation reaction which is an exothermic reaction. However, this method is not applicable to a case where the catalyst bed temperature is an ordinary temperature such as the atmospheric temperature and fuel and water are not supplied beforehand into the reaction system.                Patent Literature 1: Japanese Patent Laid-Open Publication No. 2002-158027 (U.S. Pat. No. 6,786,942)        